The present invention relates to a novel cyclone firing apparatus and to a novel method of burning ash-containing solid fuels and, in particular, to an apparatus and a method of burning lower rank coals having a high moisture content and a relatively low BTU reading, such as subbituminous class coals.
At present the most acceptable method utilized for firing coal for boilers, such as used in the electric utility industry, is that of the "cyclone" furnace. The cyclone furnace designed to burn ash-containing fuels was first introduced by U.S. Pat. No. 2,357,301. The cyclone furnace is primarily designed to utilize bituminous coals having a relatively high heat rating and low moisture content. The method and apparatus of the cyclone furnace has been further improved in subsequent patents, for example, U.S. Pat. Nos. 2,717,563, 2,731,955, 2,800,091 and 3,039,406.
In the original and succeeding patents listed above, which relate to the cyclonic principle of burning ash-containing fuels, the exit for the hot gases from the burned coal takes the shape of a cylinder or cone which protrudes part way into the combusion section or chamber of the furnace. This opening, which is known as the re-entry throat, is located at the rear of the cyclone furnace, opposite the end where the coal enters the furnace.
The above-identified prior art patents disclose a furnace structure which permits the unburned coal particles traveling along the walls of the interior of the furnace chamber away from the point of entry toward the rear of the cyclone furnace to be re-directed by the re-entry throat towards the center axis of the furnace where the hot gases therein force the carbon particles out of the furnace before they have sufficient time to completely oxidize and form ash particles which stick to the furnace walls and become part of the melted slag mass. The re-entry throat structures of the prior art patents are designed so that their outer surface flares or is directed toward the exit end of the furnace chamber.
The succeeding patents, such as U.S. Pat. Nos. 2,717,563, 2,731,955, 2,800,091 and 3,039,406, recognize several major problems which exist with the usage of such cyclone furnace structures. These problems are the incomplete combustion resulting in unburned carbon leaving the furnace chamber, an accumulation of slag in the bottom of the furnace chamber which destroys the cyclonic effect within the furnace thus compounding the slag build-up and resulting in plugging of the slag hole and excessive carry over ash and unburned coal thereby substantially destroying the effectiveness of the cyclone furnace.
These problems are accentuated by the burning of coals which have lower heat content, higher moisture and higher ash temperatures, such as are found in the Western part of the United States. For example, cyclone furnaces have been operated successfully since the late 1940's through the 1960's utilizing Illinois coal having a heat content of approximately 10,500 BTU's per pound, with a moisture content of about 12.2% and a fusion temperature of approximately 2,100.degree. F. However, when low sulfer Western coal, such as from Montana or Wyoming, having a heat content of approximately 9,300 BTU's per pound, a moisture content of approximately 24% and an ash fusion temperature of 2,400.degree.-2,700.degree. F. are fired in cyclone furnaces, because of environmental reasons, the results and burning thereof are extremely unsatisfactory.
For example, the lower BTU content of Western coal for a given fuel mass input results in the achievement of reduced combustion temperatures within the cyclone furnace. Additionally, the higher moisture content of Western coal results also in a slower burning rate and a reduced combustion temperature because a portion of the heat within the furnace is required to evaporate the moisture from the coal. This slower burning rate and further reduced combustion temperature are the major causes of improper slag formation and flow and carbon carryover. That is, unburned carbon exits the cyclone furnace to accumulate in the air heat and precipitator hoppers. Fires may result from this carbon carryover causing severe damage to the precipitators, fabric filters or air heaters. Also, the operating and maintenance costs increase significantly because much of the fuel is unburned and the amount of ash for disposal is substantially increased.
The higher ash fusion temperature of Western coals results in cyclone furnace slag tap plugging because the ash fluid temperature is not obtained and the lower combustion temperatures caused by the lower BTU high moisture content aggravate the slagging problem.
Specifically, the lack of complete combustion of Western coal particles in the cyclone furnace results in unburned carbon particles, leaving the cyclone, impinging upon the steel tubes in the adjacent boiler, thereby causing errosion damage and subsequent failure of the boiler. The unburned coal particles, leaving the cyclone chamber, are not converted to slag inside the furnace, therefore, they must be removed by external precipitators and fabric filters. When the carbon in these coal particles is not completely oxidized by the time they reach the precipitators or fabric bag filters, further damage in the form of fires may result at the precipitators or fabric bag filters.
Additionally, because a significant percentage of the heat content of the Western coal particles is not released inside the cyclone furnace chamber, a temperature sufficient to keep the viscosity of the slag at a point where it will flow into slag tap under the re-entry cone cannot be maintained. This condition is accentuated with the decreasing heat content and increasing moisture content of Western coals. When the slag tap becomes plugged, the slag accumulates in the bottom of the cyclone furnace chamber and the cyclonic action is destroyed and the slag and ash must either accumulate in the cyclone furnace chamber or exit through the re-entry cone, thus compounding the problems of operation. Accordingly, the present cyclone furnace structures in operation have a narrow tolerance of acceptable fuels and operating conditions of fuel-air ratios, which have severely restricted their commercialization.